This paper discusses the power generation expansion model CAPEX in which provisions are made that specifically account for the distinct characteristics of hydropower. The model determines trajectories of plant and capacity additions to meet the long-term electric power and energy demand at minimum discounted investment and operation costs. The problem is formulated as a mixed-integer program to account for the go-no-go construction decisions of hydro-thermal plants. The network approach is adopted to describe the system configuration and transmission lines. An integral operation routine ensures optimum and compatible operation scheduling under various conditions of power demand and availability of hydro energy for power production. Langrangian Relaxation is used to derive the solution.